This invention relates to an alloy for use in overlaying industrial components subjected to manufacturing and service conditions requiring abrasion, corrosion and galling resistance. An example of such an application is for use in coating and reconditioning steel mill caster rolls.
Steel mill caster rolls typically have a core substrate, a build-up layer, and an overlayer or stainless steel overlay. Continuous caster rolls in steel mills are overlaid with, for example, 400 series martensitic stainless steel containing, by weight, chromium ranging from 10-15%, nickel below 5%, and molybdenum around 1%. The carbon content of these steels usually varies from 0.15 to 0.2%. A typical composition for overlay alloy currently being used in the industry is Alloy 423, which has the following composition, by weight percent:
C 0.12 Mn 1.2 Si 0.4 Cr 13.5 Ni 2.5 Mo 1.2 V 0.18 Cb 0.18 Fe Balance
Overlay alloys used in reconditioning steel mill rolls are typically deposited by welding. A resulting problem sought to be addressed by the current invention is that weld deposits of these alloys are susceptible to stress corrosion cracking along interbead heat affected zones. Deposition by welding therefore increases the frequency of circumferential cracking and surface deterioration due to corrosion and wear, two common life-limiting factors in caster rolls. These problems have been correlated to thermal cycling caused by the welding process itself, subsequent service conditions in the steel mill, and carbon content of the overlay.
Thermal cycling in the welding deposition process and subsequent exposure in service can result in the formation of carbides along grain boundaries, which decreases corrosion resistance along the grain boundaries, which in turn tends to lead to circumferential cracks. In particular, Cr carbides, the majority of which are known to be the M.sub.23 C.sub.6 type (M represents a metallic ion such as Fe or Cr), develop in the temperature range of 750.degree. F. to 1300.degree. F. Because it is necessary to deposit numerous weld beads adjacent one another to cover large surfaces, thermal cycling occurs as part of an already deposited, cooled bead is heated to within the temperature range of 750.degree. F. to 1300.degree. F., resulting in the formation of carbides along the grain boundaries. Sensitization occurs as Cr-depleted regions form next to grain boundaries, lowering the corrosion resistance along the grain boundaries. During service sensitized interbead regions undergo preferential pitting corrosion attack. Pits formed in this manner act as crack nucleation sites from which the circumferential cracks initiate. The effects of sensitization can be relieved by heat treatment because carbides are known to go back into solution above 1800.degree. F. In view of the expense and effort involved in desensitizing heat treatment, however, especially of large industrial components, it would be preferable to minimize or avoid sensitization altogether.
Lowering carbon content of the deposited alloy can minimize susceptibility to sensitization by reducing the amount of C available for Cr carbide formation, but reduction in C content tends to reduce the hardness of martensite in the alloy. Lower hardness levels, due to reduced carbon, can result in poor wear and galling resistance. Attempts have been made to substitute carbon with nitrogen, although only in open arc wire deposits with limited success.
Increasing Cr content has been considered to combat sensitization, but a Cr content above 13-14% upsets the ferrite balance resulting in lower hardness, as the hardness of overlay alloys of this type is mainly governed by the ferrite balance and the C content.